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Herbicides

There are many different types of invasive plant species, often referred to as "weeds", that cause problems for agriculture producers in Alberta. Some of the more important plant species include Canada thistle, chickweed, cleavers, flixweed, scentless chamomile, stinkweed, toadflax, narrow leaved hawk's beard and wild oats. These weeds can be highly competitive, particularly if they become established before the crop. Well established weeds compete with the crop for soil moisture and nutrients. The more severe infestations can reduce yields drastically, and can constitute a significant economic loss to the farmer. For example, a light infestation of only six Canada thistle plants/square metre reduce wheat yields by 18 percent, whereas a more typical infestation of 24 plants/square metre can reduce yields by 61 percent.

Tillage used to be the principal means of controlling invasive plant species on cropland in Alberta. However, changing land management practices have seen a trend toward less tillage of the soil, and reductions in the amount of summerfallow acreage. With the availability of modern day herbicides, weed control in the agricultural sector has shifted dramatically towards chemicals combined with other control methods.

In recent years, the number of herbicide-resistant weeds and the area they infest in western Canada has increased at an alarming rate. Much of Alberta's crop area is in the early stages of one type of resistance or another. Weed resistance to a particular herbicide arises through natural selection following repeated use of the herbicide for a number of years on the same field. The best methods to delay establishment of resistant weeds include regularly changing crops and herbicides, and using plant competition, mechanical methods, and other means to augment chemical control.